Several activities important for the design, data management, and implementation of the intramural controlled trials have been undertaken during this fiscal year. In addition, broader research in controlled clinical trials has been conducted. Descriptions follow. In a cancer prevention trial, the observed treatment effect may be diluted by the presence of preexisting cases; sample size must be increased to compensate for this effect. Prescreening may reduce the number of such cases. Appropriate methods of sample size estimation and a cost model are being developed. A study to estimate the incidence of new basal cell carcinoma (BCC) in individuals already exhibiting previous BCCs was undertaken. Results from this study provide confirmatory evidence to support an estimate of the incidence of new BCCs used to plan sample size requirements for a chemoprevention trial with BCC patients. The Operations research Branch is acting as the Data Coordinating Center for the "Use of Isotretinoin in the Prevention of Basal Cell Carcinoma" chemoprevention trial. Responsibilities include all of the usual ones for a data coordinating center as well as preparation and administration of a training program for both the study coordinators and the clinical investigators, and collaboration in the interim and final analyses of study data. Pilot studies are a major component of large intervention trials, since the feasibility of many parameters needs to be tested before launching into a study with 2,000 or 20,000 patients. The "Use of Isotretinoin in the Prevention of Basal Cell Carcinoma" pilot study, completed in FY 1984 in collaboration with the Prevention Program, demonstrates some of these parameters.